To send a message

FROM : Eric PETIT
DATE : Tue Jan 01 15:15:12 2008

I am beginner in the cocoa programming and I have a crazy question.
My problem is as follows :
Inside an object (OBJSource), I want to recover the value of the 
field of another object (OBJTarget) by one assessor of this object 
(OBJTarget).
How can I reach an instance of OBJTarget inside OBJSource ?
Thanks

Eric PETIT